Magnetic disk drives and similar rotating storage devices are well known in the art. Data is stored on the surfaces of one or more vertically stacked storage disks. The data is "written" to, and "read" from the disks with one or more magnetic transducers or read/write heads. The heads are electrically coupled to signal processing electronics which provide a data transfer path between the drive and a host system. The heads are supported in close proximity to the disk surfaces by an actuator assembly, including an arm structure and a voice coil. The voice coil radially positions the arm structure in response to a control current. In rotary actuator disk drives, the movement of the heads across the disk surfaces defines an arcuate path, whereas in linear actuator files, as the name suggests, movement is linear in the radial direction.
The actuator has two distinct modes of operation. During "track seeking", the heads are moved from a current radial position on the disk to a desired or "target" radial position or cylinder in preparation for data access. "Track following" is the function of maintaining an active head in alignment with the target track while reading or writing data, or merely idling. In most disk drives today, a closed loop servo system controls the operation of the actuator assembly. During seeks, it operates to minimize overshoot and reduce random transient vibration when moving the actuator across the tracks. When track following, the loop functions to reduce repeatable and non-repeatable runout, and to counteract biasing forces. Examples of disk drive servo control systems are provided in commonly assigned U.S. Pat. Nos. 4,679,103 and 5,404,254. For a discussion of control systems in general and definitions of related terms of art, the reader is referred to a book entitled "Modern Control Engineering", by K. Ogata, published by Prentiss-Hall Corporation of Englewood Cliffs, N.J. For a discussion of general disk drive control systems during track seeking and track following, the reader is referred to "Design of a Disk File Head-Positioning Servo" by R. K. Oswald, IBM Journal of Research and Development, November 1974, pp. 506-512.
An ongoing challenge to disk drive designers has been to provide accurate servo control of the actuator. Accuracy becomes increasingly critical as the track densities of disk files increase. However, inaccuracies are introduced into the servo control system whenever actuator behavior varies from a nominal, expected performance.
Variation in actuator behavior is attributable in part to variations in the force factor (Kf) of the actuator, where Kf represents the amount of force applied to the actuator per unit of input control current. For rotary actuator files, the force factor may also be expressed in terms of torque factor, KT, or Kf*Len, where Len is the actuator arm length. The nominal Kf for a particular disk drive design is determined by such factors as the nominal strength of the field between the permanent magnets of the actuator and the number of turns of the voice coil. Variation in Kf from file to file is primarily attributable to file-to-file variations in the strength of the magnets, and to a lesser extent to the number of turns of the coil and the inertia of the actuator. Kf also varies within a single file because the strength of the magnets vary over time, and with actuator position.
The phenomenon of Kf variation with actuator position is best understood by considering the structure and operation of the voice coil motor (VCM). A rotary actuator, for example, includes an actuator arm structure pivotally mounted about a vertical axis. The actuator arms extend in a first direction from the pivot axis, and a support structure extends in the opposite direction. A voice coil is fixably mounted to the support structure between a pair of permanent magnets. Thus the coil is positioned within the fields of the permanent magnets and has a range of movement parallel to their planes. Movement of the coil is maintained within the magnetic field by crash stops or similar restrictive structures.
When a control current is applied to the VCM, a magnetic flux is generated about the coil which interacts with the permanent magnet field and produces a moving force, F, upon the actuator. Ideally, for a fixed control current, F remains constant throughout the range of actuator movement. But in reality, F varies because the strength of the permanent magnetic field varies with actuator position, r. That is, the field is the strongest along a vertical axis intersecting the centers of the magnets, corresponding to an actuator position somewhere between the inner and outer diameters (ID and OD) of the disk. It is weakest at the vertical field boundaries, corresponding to actuator positions approaching ID or OD.
Kf has also been observed to vary substantially with the magnitude and direction of the actuator control. In disk drives wherein seeks are performed by controlling deceleration, but not acceleration, of the actuator (e.g, where actuator current is at saturation during acceleration and deceleration is controlled by using a deceleration profile to slow down the actuator as it approaches the target track), the variation in current magnitude and direction is manifested as a variation in Kf with seek direction. This variation may be explained by the fact that current applied to the coil generates a magnetic field that either reinforces the field from the permanent magnets, or attenuates it, depending upon the direction and magnitude of the current. The effect becomes particularly significant during the deceleration phase of a seek operation, when coil current is very large.
Variations in Kf, whether due to radial position, magnitude and direction of the seek current, or variations in the file over time due to changing environmental factors, must be accounted for in the servo control system in order to provide accurate actuator positioning. Variations in time may be corrected by occasionally recalibrating KT during file operation.
A number of references describe servo systems having means for compensating for torque variation in a disk drive. For example, commonly owned U.S. Pat. No. 4,835,633 describes a servo control system including means for scaling the control signal to the voice coil motor to counteract positional variations in torque factor. Specifically, a scaling factor is obtained from the ratio of a nominal acceleration factor to the actual, measured torque factor of the file at a current radial position.
U.S. Pat. No. 5,305,160 to Funches et ale, describes a method for compensating for torque variations attributable to radial actuator position and component tolerances, comprising the steps of radially dividing the disk surfaces of a magnetic disk drive into a plurality of zones, testing the torque constant of the VCM in each zone with the motor moving in both directions, calculating and storing in a look-up table a compensation factor for each zone, and then retrieving and applying the appropriate compensation factor to the servo control circuitry dependent upon the zone in which the read/write heads are located during both seeks to a track and track following.
Although Funches et al. teach measuring the torque constant in both seek directions (K.sub.in and K.sub.out), the zone factor is calculated as an average of the two measurements (K.sub.ave) Thus direction of actuator movement is not addressed by their estimation model.
Published patent application JP 59-171079 to Iwai et al. describes a servo control loop wherein two look-up tables of deceleration curves (i.e., velocity profiles) are provided. One table is used when seeking in the forward direction, and the other when seeking in the reverse direction. Thus, Iwai apparently recognizes torque variation with actuator direction. However, Iwai compensates for model error introduced by directional variations.
More desirable, however, would be a closed loop actuator servo system wherein variation in torque based on seek direction is introduced in a manner such that the positioning error is reduced, rather than merely compensated for, to provide accurate actuator control.